(1) Field of the Invention
The present invention relates to a method of fabricating a thin film transistor, and more particularly to a method of fabricating a thin film transistor to improve step coverage of electrode wirings and prevent short circuit between a gate electrode and source/drain electrodes.
(2) Description of the Prior Art
Generally, a thin film transistor is used as a switching element for a device such as a flat display device, since it realizes low voltage drive, low power consumption, light weight and high quality imaging. Such a thin film transistor will be briefly described with reference to FIG. 1.
FIG. 1 is a cross section of the construction of a thin film transistor of the prior art. A gate electrode 12 is formed on a glass substrate 10, and insulating layers 13 and 14, semiconductor layer 15 and ohmic layer 16 are successively deposited thereon. Source/drain electrodes 17 and 18 come into contact with the semiconductor layer 15 through the respective ohmic layer 16, and at the same time, the gate insulating layer 14 is contacted underneath the semiconductor layer 15. A transparent conductive film, pixel electrode 19, in contact with the drain electrode 18 is formed on the gate insulating layer 14.
In the manufacturing process of a thin film transistor, step coverage of a thin film made by carrying out general vapor deposition or sputtering is one of critical problems in integrated circuit technology. A geometrical configuration of steps has an effect on each distribution of atom flux that is coated and results in a shadowing phenomenon when a thickness of a thin film deposited on a substrate is changed in thickness or electrically broken, and uneven. This shadowing phenomenon is a factor in deteriorating manufacturing yield, operation and reliability of elements. Besides, due to pinholes, when a manufactured thin film transistor is used as a switching element, a short-circuit occurs between each electrode, in a conventional gate insulating layer.
Japanese Laid Open Gazette No. 90-85826 disclosed a method concerning the formation of gate oxide film by anodization in manufacturing a thin film transistor such that a fine film is formed thereby preventing hillock formation and current leakage. A hillock is a kind of hill usually about 2 .mu.m, grown on the Al surface deposited on a Si substrate usually about 1 .mu.m. This method, however, relates only to forming an oxide film by anodization, and is not capable of improving step coverage of thin gate insulating film, and therefore problems caused thereby are still left unsolved.
As indicated above, hillocks on current leakage are avoided by the present invention. Hillock is a kind of a hill with a height of about 2 .mu.m grown on the Al surface deposited on the Si substrate of a thickness of about 1 .mu.m.
After forming a thin Al layer, hillocks can be observed. One reason hillocks grow is that the coefficient of linear expansion of silicon substrate is 3.3 ppm/.degree. C. and that of a thin Al layer is 23.6 ppm/.degree. C. The difference between the coefficients of these layers causes hillocks of Al to form on the Si substrate.
Hillocks may be divided into edge hillocks, flat topped hillocks and splice-like hillocks as illustrated in FIGS. 4(a), 4(b), and 4(c).